Automatic gain control for monopulse radar



June 28, 1960 R. A. zAcHARY AUTOMATIC GAIN coNTRoL F03 MoNoPuLsE RADAR 2 Sheets-Shea?I 1 Filed April 27, 1955 June 28, 1960 RpA. zAcHARY 2,943,317

AUTOMATIC GAIN CONTROL FOR MONOPULSE RADAR Filed April 2'7, 1955 2 Sheets-Sheet 2 um/7 2d gif/ Hr fof/JE m5 AUTOMATIC GAm AcoNTRoL Fon Y MoNoPULsE RADAR Robert A. Zachary, Utica, N.Y., assigner, by mesne assignments, to the United States of America as represented by the Secretary of the Navy I Filed Apr. 27, 195s, ser. No. 504,396

1 claim. (c1. 3434-111) This invention relates to automatic gain control circuitry for apparatus such as radar receivers and more particularly to gain control circuitry for tracking radars using simultaneous lobing from two or more antennas. Y

perimposed both in avvertical and horizontal plane. Fur- 'Y ther, if-the antennas are cantd symmetrically' in the i' horizontal plane arr-appropriate number of degrees, the

patterns diverge symmetrically about the vertical plane of symmetry. Depending upon the position of the target in the antenna pattern, returns are received differing in'phase or amplitude or both. The phase and ampli-V tude differences represent'all the information needed for obtaining the desired target spatial coordinates. Suitable detecting means are provided for detecting the sum of the two signals and the diference of the two signals when a target return is present. These sumV and difierence signals are then lead to two videoamplifiers and from there to two mixers where they beat with a standard oscillator. Thus, two channels, a sum and a difference channel V,are present in the receiver and each has its own IF amplifier requiring an automatic gain control.

Inthe practical application of this type of system considerable di'iculty is met in maintaining the amplification in the sum and difference IF amplifiers equal under all conditions of varying tube characteristics, temperature changesV and signal level. The last item, change of signal level is extremely troublesome. In the devices heretofore used an automatic gain control loop is set up around the sum Ilamplifier. to maintain the output of a second"` detector in the sum channel constant. By

means of a range gate Vthe particular pulse being tracked is separated and a proportional D.C. Vvoltage is fed back to the grids of several stages in the IF amplifier. This D C.' voltage increases or decreasesrthe IF gain in such a way asto maintain constant the `output of the IF strip for the particular pulse being tracked. The same automatic gain control voltage used for grid control in the sum IF amplifier is fed to the corresponding control grids in the difference 1F amplifier.

Although the sum and difference IF amplifiers are identical, differences in gm control characteristics of vacuum tubes make it unlikely, if not impossible, that the sum and difference IF amplifiers have identical gain control curves. The result is that if the sum and difference IF amplifiers are adjusted to have equal gains for small signal levels, their gains become unequal at high signal levels. This results in unstable or weak servo response at those signals for which the receivers are unbalanced.

V`tem receiving the returns on a two antenna simultaneous lobing radar. It is to be Vunderstood however that theY of amplifiers forming a part of a multi-channel receiving system.

It is another objectY of this invention to provide a means `for insuring equal gains ina plurality of amplies forming a part Yof a multi-channel receiving system, in spite of variation in signal level.

It is still another object of this invention to provide an automatic gaincontrol for the amplifiers in a multiy channel radar receiver wherein the gain control is con-` trolled by a .pilot signal preceding a target return.

It is a further object of this invention to provide an automatic gain control for thek amplifiers in a multi-channel radar receiver wherein a separate gain control loop isrprovided for the amplifier in each channel.v

vIt is a still further object of this invention to provide an automatic gainV control for the IF amplifiers of a multi-channel radar receiver in which a pilot vsignal is fed into each amplifier immediately preceding a target return and separateautomatic gain control loops maintain the pilot signal constant.V

A still further object of this invention-is to provide an automatic gain control circuit for a multi-channel radar receiver which also functions as noise limiting circuit.

Other objects Yand many of theV attendant advantages ofthis invention'pwill be readily appreciatedV as the same becomes better understood by reference to the following detaileddescription when considered in connection with the accompanying'drawings wherein:

.Figure l is a block diagram of the IF amplifying stage, second detecting, video amplifying stage and automatic gain control systeml embodying the invention.

Figurelisa schematic diagram of the detecting, video amplifying and automatic gain control circuits embodying the invention. l Y

vReferring now to Figurel in which is shown apportion of Aamulti-channel receiver utilizing the automatic gain control system. The invention in the disclosed embodiment herein takes the form of a two channel sysautomatic gain control system has application in a 4 or other multi-antenna simultaneous'lobing system and the following disclosure is by way of. illustration only.

A radar` [receiver incorporating Vthis invention comprises a wave guide means coupled to the pair of antennas (not shown) and a pair of first detecting meansV (not shownwyto detectY the sum of the two returns and the difference of the two returns. After amplification by -suitable amplifiers (not-shown) Ythe `sum and difference signals beat with a local oscillator pair of balanced mixers (not shown).

The sum and difference signals are then fed to a sum IF amplifier 26 and a difference IF amplifier 25 whose ,(not'shownlk in a n `gainfitris desired toeontrol automatically. This is accomplished generally by 'feeding a pilot signal consisting ofa short pulse atIF frequency to the inputs of bothV the sum and difference IF amplifiers. This pilot signal is timed to precede the transmitted pulse'of the radar by a short interval. Separate AGC loops set up around the sum and difference IF amplifiers function to maintain the pilot signal constant at the outputs of the two amplifiers.

Gain control of the radar signal is accomplished by varying the input level of the pilot signal.

Blocking oscillator 10 generates a signal to initiate i the action of a pulsed oscillator 15. This same signal is fed through a conductor 11 including a rdelay line 12 to the radar modulator so that Ythe gain controlling pulse developed by the oscillator 15 precedes the transmitted radar pulse by a short time interval. 'Ihe signal of the blocking oscillator 10 also is fed by conductor 13 to the A Patented June 2 8, .1960 y the sum and diferen'ceIl-fampliers 26 yand 25,;res'pec-L fiers aud.26the pilotI signal or'ratherltwo pilot signals lowers' 35 and 36 whichr aregco'nnected. between; a suit? fable voltagesource and the' grids of-Whichreceive a positive D.C. signal Qfrom i the .plates' of thefdetectors: Thek cathode'srof each of thecfatliocrle.followersarezcoupledrV ff;

Y ofthe 'tubesSf and` 34?; Thus, the Aoutput et tubesss'.. and 3,4"b`eing a positivel.D.C.fvoltage, 4the outputs of the cathode followersare'similarly a-positive D.C.- andact tozbiasthe,cathodes ofthe detector'slin the-desiredA man- Y ner;Y The thresholdV is determined bytheadjustmentof Y i the resistances 37,v and 38. ,-Thedeteetor tubes themselves are coupled ascathode.followershavingan output tothef i grids'39 and 40 of theamplifyingtubeslinvthejdiierence l f The system operates in thislmannerz The oscillator A1,5 f Yis being triggered'by the blockingoscillator 10 which` is also triggering the radar ymodulator through the delayl vline i2." The pilot jsi'gnal'output `Vof theloscillator 15 I Y Y therefore precedes the `.transmitted pulse' ofthe radari-by f a short time, interval.: AAfter''amplicationgby variable amplier Vi6 'andattenuation by network 25, the pilot sigl Ynals are fedto the input, of thesum andfdilerencelFf ampliers`-26 `and-252 AV.Aftl .detection v'and iiltering: a

' .plili'eriltlandl 31%?V The outputfof. the .-.ampliers 31 and '32 is therefore a: positive DLC. yvvhich is fedltio the platesV ofi.thedetectorss'land 34` so the'output. therefrom Y gated detector where it functions to gate the detector in a manner to be described in greater detail hereinafter.

The pilot signal developed by the pulsed oscillator, which is tuned'tothe intermediatefrequency is fed Ato -a l variable'gain` amplifier. lnwhich ini the form illustrated'inV 5 Figure 2 isconstituted by two amplifying ,stages V1,7,l and. 18vwith a suitable plate supply V19 and a variable tap. inductances 20 and 21, respectively wherebytheir4 gainmay loe/varied. Aconductor. 22 from the radar Y'feedsa posi. tive automatic gain control :voltage'when therange gatelocks on a radar echo thereby causingthe pilot signal to increase. r-

QThe output of.` the Vamplifier.16* is. divided between` two resistive attenuationne'tworkz to the .inputterminals of 15 tively. This is necessaryto providel isolation between these two terminals. After amplication by theamplir.

are 'detected'V bythe seconddetector.. 27 andZS andfafter. passing through suitable `ltering networks/29A and 30 are Yamplified bythe. videoampliersl amd-312..:` Y

The next stage ,comprisesthe gated detectingand D12".v g Y `output stages1 -33.and.,34. f The'trig'gergenerated by :the-f'` l blocking oscillator 10 is-'used toY gate. on the gatedldetector tubesy 33 and 34',=bei'ng fed to -`the-.gridsthereof by l25 the conductor 12. In practice the cathode of Itlleddetectors 313 and 34'-ar'e biased positively. so that-onlyfsignals arisingabove athreshold'aredetected. .Eachgated.deteci-.V 11 tor tube 33 `and"is'provided'vvitltla pair'of cathode-:fol-

through avdjustable resistances 3,7; andfSS': tofthe'cathodes d and sum IF rampliiiers respectively.r Y

e threshold voltageof thef'detectorcathodesfis-adf Vjusted so; that .in the abscnceofany AGC voltage receiverF noise-is detected; thusthe two aln'riliarygain control loops Y function as automatic noise 1evel..controls which willY hold Y receiver noisecons-tant prior to. radarlockfomzf.

2,943,317VA f g soV at the cathode is the negative D.C. for the grids of the IF amplifier. Y l

Prior to the receiving of a radar echo the noise output of each channel is held constant by the action of the gated detector sampling the noise voltageappearing above theY threshold. When the range gate;locks on a radar echo,

a positive AGC voltage proportional to the echo yfed back to the pilotsignalfamplier 16 byjmeans of theconducto 22 increasing the "grid bias 'thereoand'r causing the pilot signal to increase. Y y j t;

The Y auxiliary AGC loops around? each 1 IF amplifieraccrodingly decrease rtheig'ain ofthe sumand dife'ren'ceLIF amplifiers to hold the pilotsignal constant atlthe level de- I Ytermined by thethreshold.v Since the pilot -signal is fed to each IF ampliieratthe': sameleyel, and since Vthe l auxiliary AGC loops holdA the pilot signal outputs at Ythe same level,'the gains of the two channels remain equal.

f This is.v so,..whether thereceived radar echo `is. equal AtoY the noiseor manytimes. greater than the noisef Thus, it is seenfthat the 'invention provides/'auxiliary AGC vloops formulti-channelradar receivers rwhich loops; function as'noiselimiters andvfrnaintainthe gainsoffza plurality ofA ampliersfequal regardless of` signallevel.- y

Obviouslyl many .modications and Variations. `,of the present inventionare'possible in the light-of theaboye` teachings. '.Itis therefore to be understood that 'Withlvz the scope ofthe appended claims the invention maybeY Y Y practicedv otherwise. than .as speciii/callypdescribed. 1- Y Y An automatic .gainrco'ntrol andnoisezlimiter `V'tf'. i

for multi-channel: pulseenergyreceivers? comprising source of.pilotlsigualspmeanstriggering*said pilotjsignal Y source, delayl'in'e means lcouplingwsaid triggering meansf" f Vtot the'modulatoriof a.. pulsev energy transmitter, further meansfresponsiveto the: levelY ofn a'- pulse received by i Y receiver controlling4v the levelof' said pilot signal,A fattenu-4 ation means coupling .the outputof said pilot signal :level/1 Vf -fcontrol means Vto* iin-*amplifierV ineach-receiver channelgf rst detecting means receiving the output o,f"eachsaidiz amplifier, Ymeans .for amplifying the output offeachjde tecting means, grid. controlled vacuum tubes-havingthey` plates -thereoffcoupledto said last mentioned,amplifying." f' means, said triggering meansfbeing coupledetojtheffgridsf;

of -said tubes, means .for controlling the cathode voltage-l of each said tube, said cathodes being biasedpositivelyf.V

whereby they will conductY o nlyfuponV the simultaneous.l

imposition of a signal at the plate and a triggerapplied Y to the grid, said tubes beingcoupled as cathode followers fto VoltageY controlv elements in said'ireceiver'amplilierlchannel willbe auto-jf whereby thegain of each receiverV matically' controlled.l

j References Cited inthe l'e of this patentY VUNITED ySTATES .PA'rENXTS f sept. i212, 1119211215 Great Britain.; Oct. 5,' 1955 ir 

